MEMS mirror arrays provide an important switching engine for Wavelength Selective Switch (WSS) products. A hitless WSS requires a biaxial (tilt and roll) mirror array. Hidden hinge arrays, where the hinges are located underneath the mirror structure, are advantageous due to reduced chip size/cost, as well as improved performance, faster switching for instance. Future MEMS mirror arrays for wavelength selective switching call for relatively long and stiff or thick mirrors capable of tilting in two axes, and a relatively high tilt angle. Vertical comb drives provide relatively large electrostatic torque that is required for high tilt angle. Hidden hinges with vertical comb actuators are quite a powerful combination for next generation wavelength selective switches.
Piano-MEMS micro-mirrors, which tilt about two perpendicular axes and can be tightly packed together are disclosed in U.S. Pat. No. 6,934,439 issued Aug. 23, 2005 in the name of the present Applicant. A hidden hinge version of the piano-MEMS micro-mirrors is disclosed in U.S. Pat. No. 7,616,372 issued Apr. 4, 2007 in the name of the present Applicant. Further hidden hinge MEMS micro-mirror devices are described in U.S. Pat. No. 7,952,778 issued May 31, 2011 in the name of the present Applicant and U.S. Pat. No. 8,274,722 issued Sep. 25, 2012 in the name of the present Applicant. All four of these patents are incorporated herein by reference.
The aforementioned piano-MEMS devices pivot about a single centrally located post with the use of gimbals having torsional hinges. Generally speaking, a gimbal is a pivoted support that allows the rotation of an object about a single axis. A set of two gimbals, one mounted within the other, each with orthogonal pivot axes, may be used to provide an object mounted on the innermost gimbal two degrees of freedom of movement (e.g. tilt and roll).
When manufacturing multi-layer hidden-hinge MEMs mirrors, the hinge layer forms an intermediate layer of the MEMS so the mechanics, such as gimbals, hinges and other suspended structures, are hidden beneath the mirror. Because the hinge layer is an intermediate layer, it is very challenging to etch the hinge layer at the end of the fabrication processes. Common practice to avoid these challenges has been to etch the hinge layer mid-way through the fabrication process and complete construction of the MEMS device with freely moving gimbal platforms or other suspended structures. Accordingly, any manufacturing steps after forming the gimbals risk damaging the MEMS because of the mobility of these parts. Examples of possible damage to each MEMS mirror in the array include, interference or stiction with nearby structures during wet processing such as surface preparation prior to bonding, unintentional attachment to nearby structures during high temperature processes such as bond annealing, or failure of a hinge due to excess movement during the remaining manufacturing process.
When the hinge layer comprises one gimbal, in some embodiments the moveable platform is temporarily secured to the un-etched mirror layer's upper surface; however, the moveable platform may be of such an elongate size and shape that damage may still occur after etching the gimbal layer.
When the hinge layer comprises two gimbals, both the inner and outer platforms are free moving relative to the rest of the layer after the hinges have been etched. During fabrication, the outer platform may have a high freedom of movement in two degrees. Accordingly the risks of damage to the MEMS is increased.
Suspended structures, gimbal or otherwise, in an intermediate layer of the MEMS may result in damage during the fabrication process such as interference, failure of fragile hinges or fusion of the suspended structure to nearby structures.